1. Field of the Invention
This invention relates generally to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated block or island on the disk, and more particularly to such a disk drive with an improved clock for writing the data.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data blocks or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned blocks, the magnetic moment of the regions between the blocks must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the blocks.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as lithography and nanoimprinting from a master template or pattern. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches are recessed far enough from the read/write head to not adversely affect reading or writing. This type of patterned media is described by Moritz et al., “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736.
In patterned media, because the data islands are single-domain, the transitions between bits occur only between the islands. Since the magnetic transitions are restricted to predetermined locations governed by the locations of individual islands, it is necessary to synchronize the write pulses from the write head with the passing of individual islands under the head. Thus it is important that data island patterns in all the tracks be circumferentially aligned with one another. This alignment is established when the master template or pattern for the disk is produced. This is typically done using a rotary-stage e-beam lithography system, in which the master pattern for the patterned data tracks is produced on a track-by-track basis, or by groups of tracks at a time. Regardless of the manufacturing approach for the master pattern, drifts and offsets in the e-beam machine, which change with time, will result in unknown phase differences between the data island patterns in the different tracks. During the fabrication of the master pattern using a rotary-stage e-beam machine, drifts in the system can cause the beam position to slowly wander relative to its starting position. Since the position of the beam itself cannot be directly measured on a continuous basis, it is not possible to fully correct for all the sources of drift while the pattern is being made. The result of such drift is that as the writing of the pattern progresses from the initial track to the final track, the islands may shift by hundreds of nanometers or even many microns. Thus the data island patterns in the different tracks will be circumferentially misaligned with one another, resulting in the write pulses being out of phase with the data islands.
What is needed is a magnetic recording disk drive with patterned media that has a write-clock with phase adjustment to compensate for circumferential misalignment of the data island patterns.